Erik,
You raised an issue, 461[1], with respect to optimizing types that are
subtypes of xs:base64binary. The working group plan to amend the text of
the spec to address this issue, amongst others. Specifically in this
case, the spec will no longer talk about types in terms of value space,
but will talk about the character children of a given element being in
the canonical lexical form of xs:base64Binary. As any subtypes of
xs:base64Binary can be serialized in this form, we trust this addresses
your concern.
Regards
Martin Gudgin
For the XMLP WG
[1] http://www.w3.org/2000/xp/Group/xmlp-issues.html#x461